Forage Management
Pasture Ecology: Managing Things That We Cannot See.1
Edward B. Rayburn, Extension Specialist
October 20092
As pasture-based livestock producers we are in the business of harvesting solar energy and
converting it to food and fiber products for people. We manage plants to optimize harvesting
solar energy, animals to transfer that energy into livestock products, and cycling of mineral
nutrients in the landscape to make our businesses socially, economically, and environmentally
sustainable. We are pasture ecosystem managers.
Most of our day-to-day efforts are spent with the livestock, managing the above ground portion
of the pasture community to ensure that the livestock are properly fed. However, there is more
biomass and biological activity below ground than above (Table 1).
Let’s visit an old pasture and look at what is going on down below. We will see how this
subterranean activity helps us achieve our goals and how proper above ground management can
benefit the soil community.
Citizens of the soil community
Each plant, animal, bacteria, protozoa, and fungus has its niche or place in the pasture ecosystem.
Each has an optimum physical and chemical environment and habitat. The habitat provides
adequate food and cover, allowing the species to reproduce and maintain itself. The environment
is based on the climate, time of year, soil texture, position in the landscape, and management.
The food chain (tropic levels or food web) is composed of primary producers who fix energy
from the sun and all the other organisms that eat each other to get their part of this fixed solar
energy. These are the primary consumers, higher level consumers or predators, omnivores, and
parasites, and detritus feeders or decomposers. Here is a short introduction to the citizens of the
pasture soil community (Figure 1).
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Preseted at the Mid-Atlantic Grass-Finished Livestock Conference. October 23-24, 2009. Staunton, VA.
Programs and activities offered by the West Virginia University Extension Service are available to all persons
without regard to race, color, sex, disability, religion, age or national origin. Issued in furtherance of Cooperative
Extension Work, Acts of Congress of May 8 and June 30, 1914, in cooperation with the U.S. Department of
Agriculture.
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Plant roots are an essential portion of the above ground producers. Roots gather water and
nutrients for the plant and provide a major source of live and dead organic matter for food to soil
organisms. Legumes within this group, with their symbiotic rhizobia bacteria, fix nitrogen from
the air and provide it to the rest of the pasture community. Algae and moss are other primary
producers that provide organic matter to the community. Some algae fix nitrogen from the
atmosphere for making protein similar to the bacteria in legumes.
Earthworms eat dead plant material (detritus). They are opportunistic predators when they
consumer soil containing bacteria, protozoa, nematodes, fungus. They provide the ecological
functions of shredding large pieces of organic matter making it more accessible to bacteria and
fungus. They aerate and invert the soil, improving soil drainage, and aggregation of soil particles.
They are food for birds, moles, skunks, and carnivorous slugs and nematodes.
Table 1. Biomass of organisms above ground in the pasture and below
ground within the pasture soil.
Organisms
Standing crop biomass lbs/a
Above ground
1200 Dairy cow1 or
1200 Beef cow2
3
Live pasture
Thatch4
Below ground
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Pasture roots
Bacteria
Actinomycetes
Fungi
Algae
Protozoa
Nematodes
Mites
Collembola
Earthworms
Other fauna
587
450
1750
750
2500
2052
2052
6244
219
80
62
65
65
624
40
Below grown data largely adapted from Brady and Weil 2002.
1. Cow producing 40 lbs milk/day 180 days/acre, 50% of forage
standing crop consumed, 5 rotations/year.
2. Cow weaning 600 lb calf 3 acres/year.
3. Cool-season grass-clover pasture, 10 inches tall at grazing.
4. Thatch equal to 30% of total forage mass at ground level.
5. Assumes roots equal top growth at grazing.
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Slugs and snails are consumers of live plant material with some carnivorous species being
predators of earthworms and other slugs. They provide the service of shredding large pieces of
organic matter into smaller pieces. They are food for birds, mice, daddy longlegs, beetles, and
firefly larvae (glowworms).
Nematodes are consumers of plant roots and algae; predators of bacteria, protozoa, fungus, and
other nematodes, and parasites of insect larva, slugs, and earthworms. They provide an important
function in the nitrogen cycle by eating bacteria and releasing nitrogen back into the soil for
plants and other organisms. They are food for other nematodes, fungus, and mites.
Woodlice feed on dead plant material. They are shredders of course organic matter. They are
food for birds and spiders.
Spiders are predators. Mites are consumers of algae and predators of nematodes, springtails,
fungi, insect larva and eggs, and nematodes. Some species are detritus feeders while others are
insect parasites.
Centipeds are predators of insects, slugs, and worms. Millipeds predominately eat detritus and
fungus but some times are consumers of live plants and their seeds.
There are a number of insects that act as consumers and predators in the soil community.
Spring tails consume dead organic matter and fungus. Some consume plant seedlings, small
nematodes, and dung of other soil animals. They are food for ants.
Beetles, such as the clover root curculio, are consumers of live plants. Some beetles are predators
consuming other insects, earthworms, and snails. Other beetles, such as carrion and dung beetles,
are detritus feeders. These beetles provide the ecological function of soil aeration, soil inversion
and nutrient cycling in the community.
Ants are consumers of live plants and “farm” aphids for their honey due. They also consume
fungus and are predators of other soil organisms. They provide the function of soil inversion,
providing improved soil aeration and water infiltration.
Termites consume dead plant material and are shredders of course organic matter. They digest
cellulose using gut bacteria similar to a cow and provide the functions of soil aeration and soil
inversion.
Bacteria are central participants in the nitrogen cycle. As symbiotic consumers in root nodules
they use sugar provided by the plant as energy to fix nitrogen from the air into ammonia. This is
passed on to the plant for making protein. Other bacteria are detritus feeders and break proteins
down into ammonia while other bacterial convert ammonia to nitrate. Bacteria are food for
protozoa and nematodes.
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Figure 1. The Soil Food Web.
(NRCS http://soils.usda.gov/sqi/concepts/soil_biology/soil_food_web.html )
This chart has not included earthworms, snails, slugs, and some other soil dwelling organisms.
Actinomycetes look like fungus but are closely related to bacteria. Some species live on dead
organic matter and are able to decompose the more resistant cellulose and phospholipids. Some
species form symbiotic relations with plants and fix nitrogen while others are plant parasites.
Protozoa come in three forms; ciliates, amoeba, flagellates. Their main food source is bacteria.
They are food for nematodes.
Fungi come as molds, mushrooms, and mycorrhizae. Fungi are decomposers of dead organic
matter. Some fungi are predators of nematodes. The mycorrhizae fungi are a very important
group of fungus. In their symbiotic relation with higher plants they provide mineral nutrients to
the plant in exchange for energy and protein. Mycorrhizal fungi may cover the surface of plant
roots (ectomycorrhizal) or enter inside the plant root cells (endomycorrhizal). Fungi produce
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glomalin, a glue-like material that is essential in the formation of soil aggregates. Fungi can
decompose the more resistant organic matter such as cellulose and lignin.
Energy flow and nutrient cycles
So how do all of these citizens work together to help us manage our pastures? Their help
revolves around solar energy flow in the ecosystem and nutrient cycling.
The green leaves of plants capture solar radiation through photosynthesis. They take carbon from
the air and water from the soil and lock the energy in sugar while releasing oxygen to the
atmosphere. The plants take the sugar and add nitrogen supplied from the soil or rhizobia
bacteria if a legume (actinomycetes in some plants) to make proteins. The plants use these
sugars and proteins to grow new leaves, stems, and roots. This metabolism requires the use of all
the macro (N, P, K, S, Ca, Mg) and micro (Cu, Zn, Mn, Mo, Fe, B) nutrient minerals that are part
of the enzyme systems in the plant. Plants are the primary producers in the pasture food web.
We often talk of ecological cycles with the various mineral cycles being separate from the
carbon cycle. Carbon in the organic form is participating in the flow of energy. Other minerals
are cycled in unison with organic carbon in plants and animals, at different rates and following
different paths (Figure 2).
When a cow grazes the pasture, plants slough off some roots while they are in the process of
growing new leaves. Along with the roots clover plants drop some of their nodules. Earthworms
eat these dead roots as they burrow through the soil while bacteria decompose dead roots and
nodules that the earthworms do not eat. Later earthworms may end up consuming these bacteria
as they go back through the soil.
Some bacteria prefer the highly digestible sugars and proteins while other bacteria prefer the
readily digestible fiber; similar to bacteria in the cow’s rumen. After the really digestible parts
are digested, fungus and actinomycetes go to work digesting the less digestible fiber and lignin.
Ultimately the indigestible carbon forms somewhat stable soil humus. All of this activity is
necessary for the decomposition of dead plant material to return the mineral nutrients to the soil
to be used again by plants. This is the process of nutrient cycling.
During grazing cows also treaded down part of the pasture sward. This material likewise is
consumed by earthworms and detritus feeding insects, mites, bacteria, and fungus releasing C
back to the air as they use the carbohydrates for energy and the protein nitrogen and minerals to
sustain themselves. When organic mater energy is in good supply bacteria hold onto the nitrogen,
divide, and make a whole lot more of themselves. Then when predatory nematodes and protozoa
come alone they eat the bacteria and release a large part of the nitrogen (what that they don’t
need) back into the soil where it is available for plants to use.
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Figure 2. Generalized nutrient cycle for all mineral nutrients, carbon, and water; flow rates and
concentrations differ for each nutrient.
Harvested
Plant and
Animal
Products
Atmosphere
Management
Inputs (feeds
and fertilizers)
Plants (above
ground and within
soil)
Animals
(above ground
and within soil)
Soil and Parent Material
Surface
Water
Ground Water
Environments and niches
As we walk across the upper pasture we notice that different grasses, forbs, and legumes grow
best in different parts of the landscape. On the well drained uplands orchardgrass is abundant
while in a wet area we find a stand of reed canarygrass. These different sites present different
soil environments. The plants that do well on a part of the pasture are those adapted to the soil
chemical and physical environment in that area. They are also tolerant of the timing and intensity
of grazing placed on them by the animals as controlled by the farmer. These plants have found
their niche or place in the pasture community.
Some plants are more abundant during one part of the year and appear to be replaced by other
plants during another part of the year. For example in warm areas white clover does well in the
spring and summer while annual lespedeza is more prevalent during the heat of the summer. This
is a case of two species using the same niche but separated temporally (niches in the same
physical space but separated in time) due to environmental temperature.
The same principles apply below ground. There are many species of bacteria, protozoa and other
microorganisms that have the same ecological function in the soil. Some do better where the soil
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is high pH others do better where the soil is lower in pH. Some do best when the soil is cool
others do best when the soil is warm. Having high species diversity is good since it ensures good
microbiological activity across a range of environmental conditions.
Cows also drop dung and urine back on the soil surface. Dung is the residue of the forage that the
rumen bacteria and protozoa and acid stomach did not digest. Urine contains the nitrogen that
was excess to the cows ability to convert rumen nitrogen to bacterial protein and protein that was
excess to the cow’s need for growth and/or milk production.
Dung is quickly inhabited by dung beetles, fly larva, rove beetles that eat fly larva, earthworms,
and bacteria. One group of dung beetles lays its eggs in the cow pie while another takes the dung
and move it into burrows in the soil under the cow pie to lay eggs. A third group of dung beetles
take small balls of dung and role them away for burial in the soil as food for their larva. These
dung beetles are demonstrating different physical niches or niches separated in space. Different
species of dung beetles within these groups use the dung at different times of the year
demonstrating different temporal niches. After a while fungus and actinomycetes invade the cow
pie and help decompose the more resistant forms of carbon.
Soil Moisture a controlling factor
All of this biological activity affects plant productivity by cycling nutrients. It also has major
effects on the soil portion of the water cycle. Earthworms and dung beetles assist water
infiltration by making passageways from the surface into the lower soil. Earthworms, bacteria
and fungus assist by making glues that hold soil particles together making them stable when wet
so that the soil has more, small, stable passages for water infiltration. All of this improves water
infiltration during rain storms resulting in more water going into the soil and less running off the
surface. The organic mater and soil micropores increase the amount of plant available water the
soil can hold after a rainfall event. This allows plants to grow well longer between rains.
Nitrogen fixation and cycling
Pasture growth is highly dependent on available nitrogen. Nitrogen fixing legumes and their
symbiotic bacteria are a critical component in the pasture ecosystem. Healthy and active nodules
are identified by their red interior. This color is caused be the iron containing red leghemoglobin
used as part of the nitrogen fixing system and is similar to the hemoglobin in blood. Different
strains of rhizobium form symbiotic relations with different legume species. Legumes that form
symbiotic relations with the same strain of rhizobia or actinomycetes are grouped together as
cross-inoculation groups (Table 2). When planting legumes it is important to inoculate the seed
with the bacterial strain that will become a symbiont with the legume being planted.
In pastures both leguminous forbs and trees and their associated bacteria provide nitrogen to the
system (Table 3). Nitrogen fixed by the legume and bacteria is first supplied to the legume.
Therefore, in a new seeding the grass at first does not get much nitrogen from the legume. The
legumes take off nicely and sometimes outcompete the grasses. Legume nitrogen enters the soil
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through nitrogen rich root exudates, root and nodule death, livestock trampling, and dung and
urine deposition. This nitrogen is made available to grasses and legumes as the organic mater is
broken apart by the shredders and decomposed by the detritus feeders, bacteria, and fungus.
With time the legume roots meet with mycorrhizal fungus which attaches to the root. The
mycorrhizal fungus also attach to grass roots. Then the fungus offers the clover extra phosphate
in exchange for carbohydrate and nitrogen. The fungus provides part of the nitrogen to the grass
from which it obtained the phosphate in exchange for carbohydrates. The fungus also obtains
phosphate and trace minerals from the soil that it uses in this bartering system.
Accumulation of ammonia in the plant or high nitrate in the soil limits N fixation. Legumes
preferentially use available soil N since there is a high energy cost to fixing N. Grasses are more
competitive then legumes in taking up soil N due to their fibrous root systems. In soils that are
high in nitrate grasses will have increased N uptake and be more competitive with the legume.
This occurs in established grass-legume pastures as the organic matter builds up and a healthy
microorganism community breaks the organic matter down releasing the nitrogen and converting
it to nitrate. In these pastures grasses have a competitive advantage and the legumes may
disappear. After a period of time the soil organic matter decreases and the available soil nitrate
decreases causing the grasses to be less vigorous and the clovers come back in to the system.
This is one of the causes of clover cycles in pastures.
Table 2. Cross inoculation groupings for different species of legumes.
Cross inoculation groups
Alfalfa, black medic, bur clover, red clover, sweet clover
Birdsfoot trefoil
Cowpeas, kudzu, lespedeza, partridge pea, peanut
Crotalaria
Crown vetch
Garden beans
Garden pea, hairy vetch, Canadian field pea, Austrian winter pea
Red clover, Ladino and other white clovers, alsike clover, crimson clover
Soybeans
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Table 3. Annul rate of nitrogen fixation by different legume and none legume species
and their associated symbiotic bacteria.
Host plant
Symbiotic bacteria
Nitrogen fixed/acre/year
Alfalfa
Rhizobium
150-300
Clover, white and red
Rhizobium
100-200
Locust tree
Rhizobium
75-200
Vetch and annual legumes
Rhizobium
75-150
Alders
Actinomycetes
50-185
Kudzu
Actinomycetes
100-140
Management needed to help the system
So what are the management practices needed to help the pasture ecosystem function at its best. I
will define “best” as pasture ecosystems that captures solar energy, transfer that energy to
ruminant animals that produces food for people in a socially, economically, and environmentally
sustainable manner. These management practices are the ones that we have talked about again
and again over the years.
1.
Lime the soil to provide the pH that is optimal for plant, rhizobia, and other microbes in
the soil, a pH from 6.0-7.0 depending on the primary legume and its symbiont in the system.
2.
Apply adequate but not excessive amounts of mineral or organic P and K fertilizers to the
system. Organic fertilizers such as compost, biosolids, poultry litter, and other livestock manures
provide trace minerals (Mo, Mn, Cu, Zn, B) as well as other macro minerals (N, S, Ca, Mg).
Organic fertilizers also provide digestible carbon which is an energy source for decomposers.
This will stimulate microbial growth and activity. Soil bacteria and fungus make nutrient more
available to higher plants when they have energy and other nutrients to work with.
3.
When seeding legumes make sure to inoculate the seed with the bacteria that will become
a symbiont with the legume species being planted.
4.
Manage the nutrients already on the farm. Grazing recycles nutrients in pastures. Manure
management should recycle nutrients to the fields from which the feed was harvested. Manure
stimulates earthworm populations and activity as well as microbial populations and activity.
Organic on-farm sources of nutrients include hay, other feed wastes, manure, and bedding.
Where hay is fed back on meadows forage yields can be increased 2-fold during late spring
droughts. This is due to higher soil organic matter holding additional plant available water. In
wet years, yields can be 1.5 fold greater than where no hay was fed.
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5.
Graze pastures at the timing and intensity suitable for the forage species present. Grazing
tall grass pastures to a 2-inch residual height during cool, moist weather benefits legumes and
their rhizobia which fix nitrogen for all the organisms in the pasture system. Grazing too close
during hot droughts can be detrimental to cool-season grasses and legume.
6.
Close, rotational grazing of pastures in the fall helps develop tillers in the cool-season
grasses and stolons on white clovers. It also reduces adult clover root curculio activity in pastures
whose larvae are detrimental to legumes.
7.
Rotationally graze pastures to get good ground cover between grazing events to provide
breeding and feeding cover for night crawler earthworms. Night crawlers need adequate food and
cover for reproduction especially in the cool moist weather of the spring and fall. Earthworms,
like most other animals, prefer legumes over grasses for forage.
8.
When choosing fly control options consider their effect on dung beetles and other dung
feeding insects and organisms. There are chemical and organic methods that do no harm to these
beneficial organisms.
9.
When choosing weed control options consider their effect on other plants and soil micro
organisms. Co-grazing livestock such as sheep and goat that convert “weeds” to marketable
animal products and manure has a positive effect on the pasture ecosystem.
Above ground we manage grasses, legumes, and forbs with animals to capture solar energy,
convert solar energy into marketable livestock products, and to cycle nutrients so that our
pasture-livestock system can be sustainable. The result of our management influences the soil
environment and the organic matter available to feed macro- and microorganisms in the soil.
This affects the soil’s physical condition, availability of macro- and micronutrients to grasses,
legumes and forbs, and plant available soil moisture. Understanding how our management
affects the soil community can assist us in our management of the entire pasture ecosystem.
Further reading
Bertone, Matt, Wes Watson, Mike Stringham, Jim Green, Steve Washburn, Matt Poore, and
Mark Hucks. (no date give). Dung Beetles of Central and Eastern North Carolina Cattle Pastures,
North Carolina Cooperative Extension, North Carolina State University, Raleigh, NC.
http://www.ces.ncsu.edu/depts/ent/notes/forage/guidetoncdungbeetles.pdf
Brady, Nyle C. and Ray R. Weil. 2002. The nature and properties of soils. Prentice Hall, Upper
Saddle River, NJ 07458.
Ernst, David. 1995. The farmer’s earthworm handbook, managing your underground moneymakers. Lessiter Publications, P.O. Box 624, Brookfield, WI 53008-0624.
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Lowenfels, Jeff and Wayne Lewis. 2006. Teaming with microbes, a gardener’s guide to the soil
food web. Timber Press, Inc. 133 S.W. Ave, Suite 4550, Portland, OR 97204-3527.
Silverstein, Alvin and Virginia Silverstein. 1972. Life in a bucket of soil. Dover Publications,
Inc. Mineola, New York.
USDA/NRCS. Soil Biology Web Site.
http://soils.usda.gov/sqi/concepts/soil_biology/biology.html
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